Construct Validity and Reliability of the SARA Gait and Posture Sub-scale in Early Onset Ataxia

Aim: In children, gait and posture assessment provides a crucial marker for the early characterization, surveillance and treatment evaluation of early onset ataxia (EOA). For reliable data entry of studies targeting at gait and posture improvement, uniform quantitative biomarkers are necessary. Until now, the pediatric test construct of gait and posture scores of the Scale for Assessment and Rating of Ataxia sub-scale (SARA) is still unclear. In the present study, we aimed to validate the construct validity and reliability of the pediatric (SARA(GAIT/POSTURE)) sub-scale.

Methods: We included 28 EOA patients [15.5 (6-34) years; median (range)]. For inter-observer reliability, we determined the ICC on EOA SARA(GAIT/POSTURE) subscores by three independent pediatric neurologists. For convergent validity, we associated SARA(GAIT/POSTURE) sub-scores with: (1) Ataxic gait Severity Measurement by Klockgether (ASMK; dynamic balance), (2) Pediatric Balance Scale (PBS; static balance), (3) Gross Motor Function Classification Scale-extended and revised version (GMFCSE& R), (4) SARA-kinetic scores (SARA(KINETIC); kinetic function of the upper and lower limbs), (5) Archimedes Spiral (AS; kinetic function of the upper limbs), and (6) total SARA scores (SARA(TOTAL); i.e., summed SARA(GAIT/POSTURE), SARA(KINETIC), and SARA(SPEECH) sub-scores). For discriminant validity, we investigated whether EOA co-morbidity factors (myopathy and myoclonus) could influence SARA(GAIT/POSTURE) sub-scores.

Results: The inter-observer agreement (ICC) on EOA SARA(GAIT/POSTURE) sub-scores was high (0.97). SARA(GAIT/POSTURE) was strongly correlated with the other ataxia and functional scales [ASMK (r(s) = -0.819; p <0.001); PBS (r(s) = -0.943; p <0.001); GMFCS-E& R (rs = -0.862; p <0.001); SARA(KINETIC) (r(s) = 0.726; p <0.001); AS (r(s) = 0.609; p = 0.002); and SARATOTAL (rs = 0.935; p <0.001)]. Comorbid myopathy influenced SARA(GAIT/POSTURE) scores by concurrent muscle weakness, whereas comorbid myoclonus predominantly influenced SARA(KINETIC) scores.

Conclusion: In young EOA patients, separate SARA(GAIT/POSTURE) parameters reveal a good inter-observer agreement and convergent validity, implicating the reliability of the scale. In perspective of incomplete discriminant validity, it is advisable to interpret SARA(GAIT/POSTURE) scores for comorbid muscle weakness.